1. Field of the Invention
This invention relates to a holder for a thermal print medium, and more particularly, to such a holder for use in a thermal printer which uses lasers to provide thermal energy to the print medium.
2. Description of the Prior Art In one type of thermal printer, a dye-donor element is placed over a dye-receiving element, and the superposed elements are supported for cooperation with a print head having a plurality of individual heating resistors. When a particular heating resistor is energized, it causes dye from the donor to transfer to the receiver. The density or darkness of the printed color dye is a function of the energy delivered from the heating element to the donor. One of the problems in printers of this type is that the thermal time constant of the resistors is quite long. As a result, the printing speed is relatively slow, and the image contrast is limited.
It is known to use lasers instead of the resistors to provide the thermal energy in thermal dye transfer printing. In U.S. Pat. No. 4,804,975, for example, there is shown thermal dye transfer apparatus which comprises an array of diode lasers which can be selectively actuated to direct radiation onto a dye-carrying donor in a thermal print medium. The diode laser array extends the full width of the print medium, and radiation from the diode lasers is modulated in accordance with an information signal to form an image on a receiver sheet in the medium. A problem in apparatus of this type is in providing an intimate contact between the donor and receiver sheets during the image transfer process. It is known, as described in U.S. Pat. No. 4,245,003, to use a vacuum hold-down surface in combination with a porous receiver or receptor sheet. Vacuum applied through the porous receptor sheet is used to pull the donor sheet into contact with the receiver sheet. In many types of print medium, however, neither the donor nor the receiver is porous, and thus, the arrangement described in the patent does not solve the problem of insuring contact between the donor and the receiver sheets.